KOCHI: It was indeed heartbreaking to watch the silent fall of Vikram, the lander which came to the Moon and was on the final phase of soft-landing on the lunar surface just around 1.334 km away when Indian Space Research Organisation (ISRO) lost communication with it. According to the scientists at ISRO, the tumbling of the lander might have been due to the failure of one of the engines powering it. However, any confirmation is yet to be made by the authority concerned.
From the descending trajectory (red circle on the right panel of figure 1), it is apparent the lander underwent a severe deflection from the pre-assigned trajectory when it passed 2.1 km above the Lunar surface and beyond that point, it was almost trending and tempting at an angle closer to a vertical fall.
It can be observed that from an altitude of approximately 16.1 km to 1.334 km, the horizontal velocity has been significantly reduced from approximately 263 m/s to 48 m/s. However, the velocity reduction in the vertical component for the above-mentioned altitude range was relatively less (i.e from approximately 72.3 m/s to 60 m/s). This observation together with a high-angled descends from the trajectory likely indicate that the vertical velocity component was prominent over the horizontal one.
If we attribute the loss of communication due to the crash landing, which likely indicates a sudden acceleration of the Vikram from the deviation point. The most likely cause for this situation could be the failure of the engine as it could not generate the thrust required for its soft-landing. However, it is worth to think about other possible factors and several mysteries associated with the south pole of the Moon.
The primary force responsible for the acceleration of a falling object in the vertical component is gravity. Although the acceleration due to gravity on the surface of the Moon is negligible (just about 16.6 per cent of that on Earth), it shows large scale spatial variations with the maximum values regionally concentrating at its south pole. This means that the speed of an object falling freely at the south pole of the Moon in the absence of air resistance will increase. Since Moon is an oblate spheroid (i.e, the pole-to-pole radius is less than that at the equator), a normal increase in the force of gravity can be expected at the poles.
The yet another natural force to account for is the geomagnetism. Generally, the magnetic force will be strongest at the poles of any dipole magnetic body. However, studies indicate that the Moon currently does not have a dipolar magnetic field since its core-dynamo is no more active. However, the Moon does have a fossilised magnetic crust which was magnetised during its early evolution when the core-dynamo was active.
Therefore, an underestimation of gravity value may induce excess acceleration of Vikram lander by the neglected gravity pull at the near-surface condition integrated with the existing speed of motion of the object particularly in the absence of air resistance. Interestingly, Vikram’s landing or missing spot is well within the identified inner rim of this mass anomaly. However, as I mentioned earlier, these factors could be negligible, and may not have a major significance in the quiescent fall of the Vikram lander. But still, a cross-checking and validation of all the possible parameters would always be better and helpful for future missions.
(The author is an assistant professor with the Department of Marine Geology and Geophysics at Cusat. He is presently working as a postdoctoral scholar in a NASA project on MARS at University of Kentucky, USA)